[FFmpeg-user] ffmpeg & MPEG-2 GOP structure

[Valentin NOEL]

Thanks for your answer,

but actually the ITU-T recommendation for H.262 (ISO/IEC 13818-2, CODING OF
MOVING PICTURES AND ASSOCIATED AUDIO, 1995) specifies B-frames definition :

> *B-picture; bidirectionally predictive-coded picture: A picture that is
> coded using motion compensated prediction from past and/or future reference
> fields or frames.*
>
The "and/or" means that a only backward reference is possible.

The same document also specifies that, for closed GOP :

>
> *closed_gop -- This is a one-bit flag which indicates the nature of the
> predictions uses in the B-pictures (if any) immediately following the first
> coded I-frame following the group of picture header .closed_gop is set to
> “1” to indicate that these B-pictures have been encoded using only backward
> prediction.*
>
It also means that B-frame may use backward prediction only.

So, does that mean that FFmpeg does not allow only backward prediction for
B-frame, and that a patch is needed ?

Thanks.

2015-01-09 12:56 GMT+01:00 tim nicholson <nichot20-at-yahoo.com at ffmpeg.org>:

> On 09/01/15 10:06, Valentin NOEL wrote:
> > Hi to all,
> >
> > I would like to encode MPEG-2 video with closed GOP of 12 frames, and a
> > structure like IBBPBBPBBPBB.
> >
> > For doing this, I am using these options :
> > *-g 12* to set the GOP size
> > *-bf 2* to set the max B frames between reference frames
> > *-flags +cgop* to use Closed GOP
> > *-sc_threshold 1000000000* to disable scene change detection
> > *-b_strategy  0 *to disable adaptive number of B-frames
> > *-mpv_flags +strict_gop* to enforce GOP size (mpegvideo private flag)
> >
> > The resulting command is :
> > ffmpeg -i input.mxf -vcodec mpeg2video -g 12 -bf 2 -flags +cgop
> -b_strategy
> > 0 -mpv_flags +strict_gop -sc_threshold 1000000000 -map 0:0 -y output.mxf
> >
> > However, the result is a GOP of 12 frames IBBPBBPBBPB*P*, with an ending
> > P-frame.
> >
> > It seems the problem comes from these lines :
> >
> https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/mpegvideo_enc.c#L1491
> >
> > How can I do to fix this ? Is there a way to enforce GOP finishing by a
> > B-frame ? Is there something missing ?
> >
>
> Consider the last GOP of the file, you cannot end on a B frame as there
> is no forward frame to interpolate from.
>
> I wonder therefore if you need to add " -flags +cgop "
>
>
> > Thanks.
> >
> > Valentin
> > [..]
>
>
> --
> Tim.
> Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83
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